Curable silicone compositions that are cured by hydrosilylation reactions exhibit excellent heat resistance, cold resistance, electrical insulating properties and so on, and are therefore widely used in electrical and electronic applications. However, because cured products obtained by curing such curable silicone compositions generally exhibit high coefficients of thermal expansion, in cases where such cured products are integrated with other members, peeling occurs at the interface with the cured product as a result of temperature changes and cracks appear in the cured product upon curing. Blending a large quantity of an inorganic filler in a curable silicone composition is a well-known means for reducing the coefficient of thermal expansion of a cured product, but the viscosity of the obtained composition significantly increases, workability deteriorates, and the obtained cured product lacks flexibility.
Examples of a curable silicone composition that reduces the coefficient of thermal expansion of a cured product include following. Patent document 1 discloses a curable silicone composition comprising: an alkenyl group-containing organopolysiloxane including a straight chain organopolysiloxane, which contains 2 or more alkenyl groups among silicon-bonded organic groups in the molecule and which may comprise up to 30% of phenyl groups, with the remainder being methyl groups, and a branched chain organopolysiloxane, which contains SiO4/2 units and R(CH3)2SiO1/2 units (in the formulae, R represents an alkenyl group or a methyl group) and which contains 3 or more silicon-bonded alkenyl groups in a molecule; an alkylhydrogenpolysiloxane which comprises SiO4/2 units and R′(CH3)2SiO1/2 units (in the formulae, R′ represents a hydrogen atom or an alkyl group) and which contains 3 or more silicon-bonded hydrogen atoms in a molecule; and a platinum group metal compound. Patent document 2 discloses a curable silicone composition: comprising an alkenyl group-containing organopolysiloxane including a phenyl group-containing organopolysiloxane that contains an average of 3 or more alkenyl groups in a molecule, and a straight chain organopolysiloxane which contains 2 or more alkenyl groups among silicon-bonded organic groups in the molecule, and which may comprise 20 to 60% of phenyl groups, with the remainder being methyl groups; an alkylhydrogenpolysiloxane, which comprises SiO4/2 units and R″(CH3)2SiO1/2 units (in the formulae, R″ represents a hydrogen atom or a methyl group) and which contains 3 or more silicon-bonded hydrogen atoms in a molecule; and a platinum group metal compound. Patent document 3 discloses a curable silicone composition at least comprising: at least one type of organopolysiloxane containing at least two unsaturated aliphatic hydrocarbon groups in a molecule; at least one type of organohydrogenpolysiloxane containing at least two silicon-bonded hydrogen atoms in a molecule; a hydrosilylation catalyst; and inorganic particles which have a major axis to minor axis ratio of 1 to 1.5 and in which the proportion of particles having a diameter of 50 μm or greater is 5 wt. % or less of the total quantity of particles.
However, even these curable silicone compositions cannot adequately reduce the coefficient of thermal expansion of a cured product and cannot adequately inhibit the occurrence of cracking upon curing.